Molecular weight dependence and stereoselective chain cleavage during the early stages of the isotactic

polypropylene pyrolysis

M.C. Martíneza, R. Benaventeb, J.M. Gómez-Elvirab*

a CTR-REPSOL, Agustín de Betancourt s/n, 28935 Móstoles, Madrid, Spain b Instituto de Ciencia y Tecnología de Polímeros, ICTP-CSIC, Juan de la Cierva, 3, 28006 Madrid, Spain

http://dx.doi.org/10.1016/j.polymdegradstab.2017.06.011

Abstract

The comparative study on the pyrolysis of two Polypropylene samples, with a similar high isotactic

character but quite different molecular weights, reveals a significant difference in their thermal performances.

The detailed qualitative analysis of the parameters, that are expected to govern the thermal degradation, leads

to reasonably assume a main role of the molecular weight in the relative early stabilities. Furthermore, the

correlation found between the sharp initial build-up of the Eact and the selective split-off of isotactic

stereosequences in both samples, suggests the relevance of the stereoregular length into the energy required

for chain scission.

1. Introduction

Despite the huge effort devoted during decades to the study of thermal stability of polypropylene, there

remains a need of going deeper into the topic, because some aspects are not still well understood. This is the

case of the low initial activation energy (Eact) value of the pyrolysis and of the strong dependence of the Eact

on the conversion into volatiles, during the first stages of the process [1,2,3,4], i.e. at low and medium

temperatures.

The controversy arisen around these issues is usually bypassed in the related bibliography by resorting to

the general concept of labile structures, like chemical defect, which must be behind such behaviour. However,

the matter is not entirely solved and is not of little importance, since the understanding of the key parameters

driving such phenomena has significant technological implications. For instance in chemical recycling, where

pyrolysis offers a way of recovering valuable chemicals from polyolefin residues. In fact, the low and medium

temperature pyrolysis of polyolefins in general, and of polypropylene (PP) in particular, has a great potential

as a process for reducing the molecular weight of polymer waste, up to values that can be thus properly

introduced in fluidized bed pyrolysis reactors, where the material undergoes the eventual catalysed conversion

into chemicals [5,6]. Of course, another optional utility of the low temperature control of pyrolysis is just the

production of high added value oligomers, that can be further used as telechelics or lubricants [7,8,9,10].

The analysis carried out in a previous work on the pyrolysis of a high molecular weight isotactic PP [4],

evidenced the two foregoing facts. First, the very early Eact value of the process is abnormally low. In fact, the

initial thermal cleavage of PP chains occurs at an Eact as low as the typical value of the oxidation process

(about 100 KJ mol-1), and second, the Eact depends strongly on the weight loss during the first stages of the

process. From that study, the high molecular weight has been proposed as the main factor responsible for the

former fact, since a large chain size can produce specific intra-chain interactions that would behave as weak

points, under the severe restrictions imposed to chain dynamics in the molten state. Concerning the initial Eact

build-up with the pyrolysis conversion, the progressive molecular weight diminution was also proposed as the

main reason. The hypothesis for these facts being supported by some theoretical as well as experimental works

on the matter [11,12].

Notwithstanding the correlation found between the molecular weight and the Eact evolution with the

pyrolysis progression, the comparison of the aforementioned thermal response of a high-molecular weight PP

with that one of a substantially lower molecular weight isotactic PP, would afford enormous support on the

influence of the macromolecular character itself, into the labile nature of some regular C-C bonds, on the one

hand. On the other, this comparative study would allow checking the selective involvement of stereo-regular

chain segments into the first stages of the pyrolysis, whatever the molecular weight of the PP may be. This

specific aspect of the molecular weight reduction was proposed to explain the Eact increase, taking place from

the very beginning of the pyrolysis and up to well advanced stages. The idea is that, in the particular case of

the isotactic configuration, not only the more rigid isotactic sequences are selectively broken at the lowest

pyrolysis temperatures, but also that the length of these stereo-regular configuration drives indeed the energy

demanded for the cleavage to occur.

This study compiles therefore the data already presented before, not only to be compared with the new

ones of another isotactic PP, but to be reanalysed in more detail with the purpose of evaluating the general

validity of the explanations given and identifying other parameter with a potentially detrimental impact on the

stability of the PP.

2. Experimental part

2.1 Material and processing

The samples chosen for this study were two Ziegler-Natta highly isotactic PP (PP2472 and PP2477),

kindly supplied by Repsol. They were used as obtained from the polymerization reactor, without any additive.

The high molecular weight specimen (HPP) was synthesized in the absence of hydrogen, while the low

molecular weight one (LPP) was synthesized under 12 bar of hydrogen. The molecular weights, as they have

been obtained by viscometry (section 2.3), are collected in table I. Their respective melt index values are

shown in table I. The powdery samples were processed by hot pressing at 190 ºC and 20 bar, for 2 min, and

quenched at room temperature to yield films, which were finally used for both analytical and preparative

thermo-gravimetric experiments.

Table I. Characterization of the samples, Mw, melt index, crystallinty and melting temperature of the

quenched samples from the melt

Sample Mw

fc

(wt%)

Tm

(ºC)

Melt index

(g·10 min-1)

HPP 940,000 75 162.4 0.12

LPP 195,000 62 163.0 23.35

2.2 Dynamic pyrolysis

The pyrolysis of the polymer films was carried out by running analytical dynamic TGA as well as in

preparative mode, by using a previously described ad-hoc pyrolysis set-up [3,4]. Thus, large enough quantities

of dynamically pyrolysed samples could be obtained to perform a complete characterization of the pyrolysed

polymer, all along the process. The two series of pyrolysed samples are detailed in table II.

The analytical TGA measurements were carried out from 40 ºC up to 750 ºC in a Thermal

AnalysisTGA-Q-500 device. The samples used were 4mm discs of approximately 5 mg cut from the film with

a calibrated die. Constant heating rates of 2, 5, 10 and 20 ºC/min under a 50 mL/min flow of Helium were

used.

Table II. Series of pyrolysed samples

Final

temperature

of pyrolysis

(ºC)

HPP

(Weight loss %)

LPP

(Weight loss %)

200 0.1 0.1

250 0.2 -

300 - 0.36

330 0.7 -

350 4.7 2.1

375 12.8 5.75

385 15.6 -

390 29.8 10.9

400 40.4 23.2

410 59.0 43.3

420 90.1 68.6

In the case of preparative dynamic pyrolysis, the experiments were run from initial quantities between

1.0 and 1.3g of the film. The heating ramp inside the pyrolysis cell was checked to correspond to the

programmed 2 ºC/min rate in the furnace, by means of a K type thermocouple/data logger (Lascar Electronics),

which tracked the actual temperature value every 10 s. After a 15 min purging stage with Nitrogen (250

mL/min), the pyrolysis cell was let equilibrate at 40 ºC and, then, the heating ramp was started in the Nitrogen

stream. Finally, the sample was cooled down under N2 at room temperature. The weight loss was estimated

by difference weighing. All samples were purified by dissolving them in xylene at 110 ºC under N2 bubbling

and final precipitation in ethanol. After washing with ethanol and drying at room temperature under vacuum,

a white polymer residue was obtained in all cases. The purification step assures the removing of impurities,

i.e. volatile oligomers, which can stay into the polymer bulk, especially at the highest pyrolysis temperatures.

2.3 Viscometry

The weight average molecular mass of both virgin PP films, as well as those ones of samples pyrolysed

up to 330 ºC and 350 ºC, for HPP and LPP respectively, were estimated by means of the intrinsic viscosity in

tetralin stabilized with Irganox 1010 (1 g/L), at 135 ºC, by using the relationship [] = 1.66·10-4 · Mw0.733 [13].

Viscometry is not suitable for low molecular weights of samples pyrolysed beyond the abovementioned

temperatures; hence, the 1HNMR analysis was employed in these cases to obtain the number average

molecular weight (section 2.6). In the case of sample HPP pyrolysed at 330ºC and sample LPP pyrolysed at

350ºC, both viscometry and 1HNMR analysis could be used to get the weight and the number average of the

molecular weight.

2.4 FTIR characterization

The FTIR spectra of the virgin HPP and LPP films were recorded by performing 10 scans in a Perkin-

Elmer Spectrum One, in the 4000 to 450 cm-1 range, at a resolution of 2 cm-1. The spectra have been base line

corrected and normalized to the 2722 cm-1 peak.

2.5 Thermal analysis

The thermal properties were analyzed in a TA Q100 Instrument calorimeter connected to a cooling

system and calibrated with Indium and Zinc. The films of the samples were heated from -45 º C to 220 ºC at

a heating rate of 20 ºC/min. The values of Tm and crystallinity (fc) were taken from the maximum and the

enthalpy of fusion, respectively, of the endotherm peak obtained. For the estimation of the crystallinity, a

value of 162 J/g [14] was taken as the melting enthalpy of a 100% crystalline iPP. The results are shown in

table I.

2.6 NMR characterization

Chain microstructure of samples, i.e. molecular weight, tacticity and anomalous groups produced

during the pyrolysis process, was analyzed by means of 13C and 1H NMR spectroscopy. In the former case,

the spectra were obtained from 1,1,2,2- tetrachloroethane-d2 (10 wt%) in a Bruker Avance III HD (125 MHz)

at 100ºC, using the solvent signal at 74.00 ppm as an internal reference. A minimum of 8000 scans were

recorded with broad band proton decoupling, an acquisition time of 0.59 s, a relaxation delay of 5 s and a

pulse angle of 30º. The 1H NMR spectra were previously recorded in the same spectrometer, at the same

temperature, in order to accurately assess the relative content of chemical groups generated during the

pyrolysis. This way, any transformation due to long residence times at high temperature in the 13C NMR

analysis is thus prevented.

While 13CNMR has been employed to measure the evolution of the tacticity distribution with the

pyrolysis temperature [3,4], 1HNMR provides accurate results on the chain-end group content and,

consequently, on the molecular weight from about 50,000 dalton downwards. Terminal double bonds analyzed

include vinylidene, vinylene and isobutenyl groups. The structures of these species, their characteristic signals

and the details about the calculus of the molecular weight are reported in previous works [3,4].

3. Results and discussion

Non-isothermal methods (iso-conversional or not) have been used for long in the estimation of the Eact of

polymer degradation, by using the TGA technique, either under oxidative or inert atmosphere

[1,2,15,16,17,18]. More recently, a comparative study on the thermal and thermooxidative stability of several

polymers, performed by TGA, DSC and CL [19], has shown that the multiple heating rate is the recommended

approach for the Eact assessment, especially when the TGA is used, even though the sensitivity of the loss

volatiles measurement is much lower than that one of the CL signal. Despite this, some care is needed when

interpreting TGA results in terms of the energy required for chain bond scission to take place. In fact, values

of the Eact obtained at very high weight losses can be influenced by some other processes than the mere chain

scission. This is the case, for instance, of the volatilization of relatively high molecular weight molecules,

either in the form of radical or neutral species. Consequently, although the Eact has been calculated all along

the complete weight loss, only conversions up to 50% have been considered in order to withdraw consistent

conclusions.

The TGA curves for the two PP samples studied, obtained at the different heating rates, are shown in figure

1. All of them correspond to one-single step processes that can been described through an apparent first order

mechanisms, where the only variable is the mass conversion into volatiles [1,15,19]. This feature can be also

remarked in the DTGA curves shown in figure 2, where one single peak is detected. The most important

difference between the two PP samples is the quite different stability found at low heating rates. It is absolutely

clear that the lowest molecular weight PP exhibits higher values of both Tonset (figure 1) and Tpeak (figure

2), especially at 2 ºC·min-1 (see table I) and at low and medium conversion degrees (roughly below 50 weight

loss %). On the contrary, an inversion of the thermal stability is found at 10 and 20ºC·min-1.

Figure 1. Series of TGA curves for the HPP (black) and LPP (blue) films at 2 (solid thick lines), 5 (solid thin

lines), 10 (dashed lines) and 20 ºC/min (dotted lines).

Figure 2. Series of the dW/dT curves for the pyrolysis of the HPP (black) and LPP (blue) samples at 2 (solid

thick lines), 5 (solid thin lines), 10 (dashed lines) and 20 ºC/min (dotted lines).

The fact confirming that the thermal stability of the two PP samples is really different, is what it is found

when the weight loss is tracked during the preparative pyrolysis. Figure 3 displays the weight loss evolution

at 2ºC·min-1 in both samples, under nitrogen, and the corresponding analytical TGA curves at the same heating

rate. It is quite apparent that preparative and analytical curves are not that different and then, that the two

specimens reflect an identical qualitative behaviour under these different conditions, being the low-molecular

weight PP more stable. The inspection of the pyrolysis progression in both polymers, shown in table II, reveals

that the weight loss reached by LPP is about a half of that one of HPP, just from low conversions at 350ºC,

and this difference keeps up almost constant for conversions under 50 wt loss %.

Figure 3. Comparison of the weight loss at 2 ºC/min for analytical (lines) and preparative (circles) TGA

runs. Data for HPP are in black and in blue for LPP.

On account of the fact that little differences between the two samples, arising from distinctive

characteristics of the PP samples, can make the development of the early stages of pyrolysis to be kinetically

different, it is imperative to consider, at least from a qualitative point of view, which influence would be

expected for each one of them.

Some data in related literature [19] have evidenced the influence of air traces adsorbed in the amorphous

phase in the initial degradation states of pyrolysis. For this reason, this study has been carried out with films

obtained under same processing conditions (see experimental part). Furthermore, the crystallinity of the

samples has been compared in order to assess the fraction of amorphous phase, where air might be trapped at

a molecular level. The results obtained by means of the DSC are shown in table 1 and allow reasonably

concluding that LPP can retain a higher oxygen content and, consequently, its thermal stability should be

lower than that one of HPP. However, the actual fact is that LPP is more stable.

The difference in melt viscosity is a second parameter that must be taken into account. The large difference

in the melt index of both samples (table I) is expected to make the loss of volatiles more difficult in HPP and,

then, to show this sample as apparently more stable. However, the reverse fact occurs.

Other factor that could be involved is the quantity and/or quality of anomalous groups, mainly oxygen-

containing functions and double bonds. On this issue, the role of terminal double bonds in the different thermal

stability is ruled out, because of the smaller content of such groups in the highest molecular weight HPP. In

addition, the fact that the Eact increases in the initial states of pyrolysis, despite the continuous double bond

build-up, allows attributing a low relevance to these species as a pro-degradant under pyrolytic conditions.

A second type of anomalous groups are oxygen-containing chain units. In this case, a meticulous

inspection of the very small 1HNMR signals, i.e. those ones whose intensity is equal or below the olefinic

protons in the virgin polymers, reveals that the only peak with a vanishing evolution with pyrolysis conversion

seems to be that one at 2.15 ppm, that can be tentatively assigned to methyl ketone groups [20]. This is what

can be inferred from the evolution of such signal for HPP and LPP in figures 4 and 5 respectively. In both

figures, the increasingly intensity of the vinylidene signals at 4.73 ppm, 4.81 ppm and from 2.10 to 2.15 ppm,

allows tracking the advance of the pyrolysis. Certainly, the removal of methylketone groups seems to occur

at low conversions but, once again, the results contradict what it would be expected if these carbonyl species

had a great impact in the very early thermal stability of the samples. Actually, the less stable HPP sample

shows a lower content of these carbonyl species than the LPP sample, as it can be also confirmed by comparing

the normalized FTIR windows corresponding to the CO stretching region. This can be checked in figure 6,

where the signals of CO groups of either saturated ketones and acids (1708 cm-1) or α,β-unsaturated ketones

(1688 cm-1) are more intense in the normalized spectrum of LPP.

Figure 4. Evolution of the 1H NMR spectral window of protons associated to vinylidene and methylketone

end-capping groups, for HPP, with the weight loss %.

Figure 5. Evolution of the 1H NMR spectral window of protons associated to vinylidene and methylketone

end-capping groups, for LPP, with the weight loss %.

Figure 6. FTIR spectral window corresponding to the CO and C=C stretching modes of samples HPP

(black) and LPP (blue).

Finally, the analysis of the hydroperoxy groups by FTIR (figure 7), both associated (3410 cm-1) and free

(3550 cm-1) [21], does not allow either ascribing the lower thermal stability of HPP to a higher content of

these species. In fact, their contents are similar in both samples.

Figure 7. FTIR spectral window corresponding to the R-OOH stretching modes of samples HPP (black)

and LPP (blue).

The evolution of the Eact in both samples with the pyrolysis conversion is displayed in figure 8, as it is

calculated from the Friedman method [22]. This figure also shows the exponential decays of the corresponding

molecular weights. Even though the pyrolysis mechanism is unique and the two trends are qualitatively similar

in the weight loss range of interest (see figure 9 showing the normalized trends), both samples show

remarkable differences at low weight losses. In particular, the very initial Eact of the process is much lower in

HPP than in LPP and this difference is gradually reduced as pyrolysis progresses. The Eact becomes pretty

much similar when molecular weights of both samples are of the same order, at about 5 wt.loss%. These trends

of the Eact appear more detailed in figure 10, where the molecular weight is added as a second variable. In this

case, the plotted Eact values are what should be expected for prepared samples (table II), according to

extrapolations based on curves shown in figure 8. In such figure, the projection of the 3D curves on the

Eact/Mw plane illustrates how the above mentioned convergence of the Eact takes place in a roughly unique

linear way, once the pyrolysis has left the initial stages.

Figure 8. Evolutions of Eact (solid circles) and molecular weight (open circles) with the weight loss %. HPP

(black), LPP (blue)

Figure 9. Evolutions of Eact with the weight loss % in a normalized scale. HPP (black), LPP (blue)

To sum up, two main results are well worth standing out. Firstly, despite the fact that the mechanism of

the PP is unique, there is a significant difference in the Eact of processes occurring at the very beginning of the

pyrolysis, i.e. when the molecular weight difference in both PP samples is very important. Secondly, the Eact

reach similar and increasingly values for further conversions, when molecular weight become similar.

As far as the initial Eact gap is concerned, some causes have been argued to account for it without reaching

a definitive conclusion, because of the practical difficulties in evidencing their influence. The most invoked

one has been the presence of very low contents of anomalous groups in the virgin [1,2,23] polymer, as it is

obtained from the polymerization reactor. On this matter, the recent work of Tobita et al [24] is a smart way

of proving that some chemical defects are inevitably produced during the propene polymerization. In

particular, the content of foreign groups seems to be closely related to the catalyst activity and, therefore, to

very high local temperatures in the polymerization medium, which would be ultimately the reason for the

production of anomalous chemical structures.

Figure 10. Inter-relationship between Eact, molecular weight and weight loss %. HPP (black), LPP (blue)

Our results point out indeed to the involvement of carbonyl species in the initial stage of the pyrolysis and,

therefore, these groups must be considered as low Eact defects playing a role in the initiation. However, it is

clear that in the highest molecular weight sample there must be another reason able to explain such a low

initial Eact value, as for it has the smallest CO content. That reason is tentatively associated to the large

molecular weight of HPP, since the Eact difference is apparent as long as chain size of both samples holds

fairly different. This happens for pyrolysis levels below 5 wt.loss% as it was indicated in figure 10, while for

further weight losses, the molecular weight of HPP even drops down under the level of LPP. This is better

illustrated in figure 11 where the inversion of the molecular weight trends can be observed. This phenomenon

is obviously related to the occurrence of chain scissions and then to the increase of chain-end double bonds.

The representation of the diminution of the molecular weight, normalized to the initial value, together with

the increase of terminal double bonds in figure 12, makes clear that chain scission is initially faster in HPP

and that it is the decisive factor determining the relative evolution of both molecular weights.

Figure 11. Evolution of the molecular weight with the final pyrolysis temperature for HPP (black) and LPP

(blue): solid circles correspond to weight averages from viscometry and open circles to number averages

from 1H NMR.

Figure 12. Evolutions of the molecular weight (normalized for values from viscosity) (solid circles) and

terminal double bond content (open circles) with the final pyrolysis temperature: HPP (black) and LPP

(blue)

On the basis of the above commented considerations, the high molecular weight is proposed to be in the

origin of the lower initial thermal stability of HPP with respect to LPP. The higher chain size of the former is

expected to generate more severe restrictions to chain dynamics in the molten state by producing intra-chain

interactions which, under high thermal stress, would behave as weak points. The work of Nyden et al [11] on

the simulation of the iPP pyrolysis predicts indeed an initial value for the Eact as low as 100 KJ·mol-1 and

points out to an effective role of the macromolecular character itself in the initiation of pyrolysis. In our case,

considering that the isotacticity degree is similar in both samples, the especially high molecular weight of HPP

is a factor that would justify a lower Eact value.

On the other hand, the aforementioned authors have also shown how the Eact of the poly-isobutylene

pyrolysis increases when the molecular weight diminishes [12], because of the removal of non-bonded

interactions in polymer chains. The loss of the macromolecular character of chains makes the Eact to gradually

approach the theoretical value corresponding to the C-C dissociation energy. This is what it is found with the

pyrolysis progression in both PP samples. Anyway, the measured Eact values must be considered to reflect the

steric characteristic of links between propylene units that, in our case, is isotactic in both samples. In fact, the

Eact evolution of the more flexible syndiotactic PP chains has been reported to be appreciably over the values

presented here [3,25]. A difference that is ascribed to the energy consumption needed for the distinctive GGTT

helix in sPP chains to be uncoiled, before the C-C scission occurs [25]. On the contrary, the ability of the

characteristic GTGT helix to dissipate energy by deformation is quite limited. Therefore, isotactic chain

segments are expected to be especially prone to be broken over the temperature build-up.

This hypothesis can be assumed from the detailed evaluation of the tacticity distribution at the pentad level

in the iPP samples, all over the conversion range. Figure 13 reveals indeed, that the production of terminal

double bonds takes place at the expense of C-C bonds in (mmmm) pentads exclusively, while the content of

any other pentad does not change in the initial conversion range (only the total of the (non-mmmm) pentads

has been plotted for the purpose of clarity). This happens up to a content of roughly 2 mol% of terminal double

bonds, which corresponds to 23 and 30 wt.loss % for LPP and HPP respectively. Within these initial weight

loss periods, the molecular weight of both PP samples has already collapsed up to values in the order of a few

thousand Dalton (4600 and 3700 for LPP and HPP respectively), and the Eact is about to leave the initial stage

of fast growth. This finding suggests that the increase of the Eact could be related with the variations in length

of the isotactic segments involved, in such a manner that the shorter the isotactic length is the more difficult

the chain scission would occur. A hypothesis that needs confirmation by additional work.

Figure 13. Evolution of the mmmm (solid circles) and non-mmmm (open circles) pentad contents with the

percentage of terminal double bonds. HPP (black), LPP (blue)

4. Conclusions

Despite the fact that the two PP studied exhibit similar values of isotacticity, their thermal stabilities are

proven to be quite different. The negative impact of some factors like oxygen adsorbed in the amorphous

phase and the physical restrictions imposed by the viscosity of the molten state for the volatiles to evolve, are

inferred not to be important, as for they would contribute the most in the more stable LPP specimen.

In addition, although methylcarbonyl groups are likely involved in the very early stages of the pyrolysis,

they do not explain the particularly low values of both the Tonset and the initial Eact of the HPP sample. On

account of the fact that the largest difference between both PP lies in their molecular weights, a very large

chain size is proposed to produce an enhanced content of specific chain interactions in the molten state, which

would behave as weak points under high thermal stress. Bringing to mind again the secondary role of other

factors, the higher thermal stability of the low molecular weight sample suggests that a shorter average chain

length is the main cause of a higher performance, since the likelihood of producing weak points by intrachain

distortions is lower.

The further Eact increase with the progression of the pyrolysis, can be related not only to the diminution of

the molecular weight, as some theoretical works predict, but to the decrease of the length of isotactic segments

where C-C scission selectively occurs. The linear relationship between the vanishing of (mmmm) pentads and

the initial build-up of terminal double bonds, i.e. during the diminution and collapse of the PP molecular

weight, without changing that one of the (non-mmmm) pentads, supports this hypothesis.

Acknowledgements

The authors acknowledge the financial support of MINECO Spain (project MAT2016-79869-C2-1-P), as

well as the kindly assistance of Repsol for the supply of samples.

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